I have always beleaved that the hole at the top of vintage set of cleaning rods below the top threaded hole that at that time would hold a Turk's head brush rather than a jag especially if the hole has a tapered enterence, it was used to hold a piece of "Flax or Cotton Tow" used for cleaning or oiling the bore.

The only lessons in my life I truly did learn from where the ones I paid for!